At 5:00 a.m. EST on January 20, the most important alarm clock in the solar system will wake up the European Space Agency’s (ESA) sleeping Rosetta spacecraft.

Rosetta is chasing Comet 67P/Churyumov–Gerasimenko and, since its launch in 2004, has made three flybys of Earth and one of Mars to build up enough speed and get on a trajectory toward the comet. It has also encountered asteroids Steins and Lutetia along the way.

Operating on solar energy alone, the spacecraft was placed into a deep space slumber in mid-2011 as it cruised far from the Sun and out toward the orbit of Jupiter. To prepare for its long sleep, Rosetta was oriented so that its solar arrays faced the Sun and put into a once-per-minute spin for stability.

The only devices left running were its computer and several heaters.

Now, 31 months later, Rosetta’s orbit has brought it back to within 418 million miles (673 million kilometers) of the Sun, and there is finally enough solar energy to power the spacecraft fully again. It is time to wake up.

Rosetta’s computer is programmed to carry out a sequence of events to re-establish contact with Earth on January 20, starting with an “alarm clock” at 5:00 a.m. EST.

Immediately after, the spacecraft’s startrackers will begin to warm up, taking about six hours.

Then, its thrusters will fire to stop the slow rotation. A slight adjustment will be made to Rosetta’s orientation to ensure that the solar arrays are still facing directly toward the Sun before the startrackers are switched on to determine the spacecraft’s attitude.

Once that has been established, Rosetta will turn directly toward Earth, switch on its transmitter and point its high-gain antenna to send its signal to announce that it is awake.

Because of Rosetta’s vast distance — just over 501 million miles (807 million km) from Earth — it will take 45 minutes for the signal to reach the ground stations. The first opportunity for receiving a signal on Earth is expected between 12:30 to 1:30 p.m. EST.

Deep space tracking dishes will be listening for the signal, starting with NASA’s “big ears” — the 70-meter-diameter station at Goldstone, California, followed by, as Earth rotates, the Canberra station in eastern Australia. ESA's New Norcia 35-meter antenna in Western Australia would be next in line to await the signal's arrival.
Whenever the signal is received, it will be relayed immediately to ESA’s Operations Center in Darmstadt, Germany.

Once mission controllers have verified Rosetta’s health, each of its scientific instruments will be switched back on and checked out, an effort that will take several months as the spacecraft continues to eat up the remaining 5.6 million miles (9 million km) separating it from the comet.

In May, Rosetta will make a major maneuver to line up for arriving at its target comet in August. If all goes well, it will become the first space mission to rendezvous with a comet, the first to attempt a landing, and the first to follow a comet as it swings around the Sun.

Comets are considered to be the primitive building blocks of the solar system and likely helped “seed” Earth with water and perhaps even the ingredients for life. But many fundamental questions about these enigmatic objects remain, and through its comprehensive close-up study of Comet 67P/Churyumov–Gerasimenko, Rosetta aims to unlock the secrets within.